1. Field
The technology described herein relates to oscillators providing oscillating signals having arbitrary frequencies and to systems and methods for using the same.
2. Related Art
Oscillators are ubiquitous components in electronic equipment including wireless and wireline communications systems, entertainment electronics, aerospace systems, and timing systems. The oscillators traditionally are used to provide a reference signal or clock signal, such that precision of the signal frequency is important. Conventionally, crystal oscillators having quartz crystals as the resonating element have served as the oscillators of choice because they can be manufactured to provide precise signal frequencies within ±1.5 parts-per-million (ppm) of a target frequency value, frequency stabilities of ±2.5 ppm over the entire operating temperature range from −40° C. to +85° C., aging of below ±1 ppm/year (at 25° C.), typical phase noise of −138 dBc/Hz at 1 kHz, and power consumption as low as 1.5 mA.
Standard frequencies for reference signals and clock signals have developed, and oscillator manufacturing has conformed to these standard frequencies. Typical frequency values are as low as 32.768 kHz for watch crystals and real time clocks. Frequencies in the MHz range are commonly used in cell phones and GPS receivers, including 12.6 MHz, 13 MHz, 14.4 MHz, 16 MHz, 16.368 MHz, 16.9 MHz, 19.2 MHz, 19.8 MHz, 20 MHz, 23.104 MHz, 24.554 MHz, 26 MHz, 27.456 MHz, 32 MHz, 33.6 MHz, 38.4 MHz, and 52 MHz. Owing to the ability to manufacture quartz crystals to provide a precise target frequency, it is conventional for crystal oscillators to be manufactured to provide one of the several standard frequencies.
Thus, circuits and systems including crystal oscillators or receiving signals from crystal oscillators are conventionally designed to work with one of the standard frequencies corresponding to the particular crystal oscillator being used. FIG. 1A illustrates a conventional apparatus 100 including an oscillator 102 and system 106 that receives at its input port 105 an oscillator signal 104 output from an output port 103 of the oscillator 102. The system 106 is designed to work with a signal of precisely 26 MHz. Therefore, a 26 MHz oscillator is selected for the oscillator 102. If the system 106 receives a different frequency, it will not operate properly.
In some conventional devices, circuitry is designed to operate with a frequency other than that provided by the oscillator, but which can be precisely generated from a known, precise oscillator frequency conforming to one of the standard frequencies. Referring to FIG. 1B, the apparatus 150 includes the previously described oscillator 102 and a system 156, which itself includes a frequency synthesizer 158 and a sub-system 162. The sub-system 162 is designed to operate with a frequency other than the 26 MHz of oscillator signal 104 provided by the oscillator 102. The frequency synthesizer receives the oscillator signal 104 at its input port 155 and generates a synthesized signal 160, which can be referred to as an internal signal since it is generated and used internally to system 156, having the frequency required by sub-system 162. If the synthesizer 158 does not receive a precise 26 MHz signal from the oscillator, it will not generate the precise frequency required by subsystem 162, and therefore the subsystem 162 will not operate properly.
In the event that an oscillator does not provide a frequency precisely matching that required by a system, some conventional devices include circuitry to provide a tuning signal to the oscillator, referred to as automatic frequency control (AFC), as shown in FIGS. 2A and 2B. The apparatus 200 of FIG. 2A includes a 26 MHz oscillator 202 which provides the oscillator signal 104 to a system 206. Although the oscillator 202 is shown as a 26 MHz oscillator, for conventional oscillators the oscillator signal 104 can differ from 26 MHz by ±2 ppm. The system 206 determines whether the oscillator signal 104 has a frequency of precisely 26 MHz, and includes an output port 208 from which is provided an AFC tuning signal 210 to tune the oscillator if the oscillator signal 104 is not precisely 26 MHz. The AFC tuning signal 210 is received at an electronic frequency control input port (EFC_tune) 204 of the oscillator.
In FIG. 2B, the apparatus 250 includes the 26 MHz oscillator 202 and a system 256 having an input terminal 255 to receive the oscillator signal 104. The frequency synthesizer 258 generates a synthesized, or internal, signal 212 which is provided to the subsystem 262. The subsystem 262 detects whether the synthesized signal has the precise frequency required for proper operation of the sub-system and provides, via output port 264, the AFC tuning signal 210 to tune the oscillator 202 if the frequency of synthesized signal 212 does not precisely match the required frequency.
Conventional AFC tuning is limited to ±30 ppm of an initial frequency by the properties of the quartz crystals used as the resonating elements of conventional crystal oscillators, and is typically limited to ±10 ppm in practice.